1. Field of the Invention
The present invention relates to an apparatus and method for correcting the white balance of an image signal.
2. Description of the Related Art
In photographing devices, such as digital cameras and the like, it is well known that white balance corrections are performed on subject images after they are converted to image signals by the photographing device.
Japanese Unexamined Patent Publication No. 6(1994)-121332 discloses a picture signal processing system that detects a skin color region or a human face from picture signals and performs luminance correction, color correction, and aperture correction only on the detected region. That is, it discloses a technique for improving skin color reproducibility by regarding a skin color region in picture signals as a face region and performing a color correction on the face region. According to the disclosed technique, the improvement of skin color reproducibility is realized by outputting a skin color detection signal representing a skin color region from a comparator according to memory output which is based on the phase and amplitude of R-Y, B-Y and color difference signals, and the level of luminance signal, and controlling the gains of the R-Y, B-Y and color difference signals by a gain control circuit based on the skin color detection signal.
U.S. Patent Application Publication No. 20060171605 discloses an image processing system that extracts the shape of a main subject from image data, then determines a plurality of regions including the region corresponding to the shape of the main subject as weighting regions, and performs exposure correction and white balance correction based on the photometric value, colorimetric value and weighting factor of each of the weighting regions. According to the disclosed technique, the shape of a main subject, such as a person or the like, is extracted from the original image data, then the region of the main subject corresponding to the shape of the main subject is determined as a region for the photometry/colorimetry, a plurality of regions including the main subject region is determined as weighting regions, and a weighting factor is allocated to each of the weighting regions. In the mean time, the entire image plane is divided into a plurality of segments, then a measured value of segment photometry and/or colorimetry is calculated for each of the segments, and the weighting factor is determined for the measured value calculated for each of the segments. Thereafter, exposure correction and/or white balance correction is performed based on the measured value calculated with respect to each of the segments and the weighting factor determined with respect to each of the weighting regions to which the segment having each of the measured values belongs.
Japanese Unexamined Patent Publication No. 2003-189325 discloses an image processing system that realizes favorable image processing by performing face detection and avoiding the use of color information of the face portion in the calculation of white balance to prevent unpleasant white balance causing the face portion to become pale or the like. According to the disclosed technique, the prevention of unpleasant white balance is realized in the following manner. First, a determination is made as to whether or not an imaging device is in face recognition mode, and if it is not in the face recognition mode, the AWB evaluation value obtaining area is specified to the normal area. If it is in the face recognition mode, a face recognition process is performed and a determination is made as to whether or not a face is recognized. If a face is not recognized, the AWB evaluation value obtaining area is specified to the normal area, while a face is recognized the AWB evaluation value obtaining area is moved to an area other than the face. Then, the AWB evaluation value obtaining area is set to obtain an AWB evaluation value and a final AWB gain is set by calculating the AWB value.
The methods described above devise ways to obtain a better image, in particular, for color correction. The technique described in Japanese Unexamined Patent Publication No. 6(1994)-121332 is a technique that detects only a skin color portion for color correction in order to improve human skin color reproducibility. It may improve the skin color reproducibility, but it does not consider a portion other than the skin color portion. Consequently, this might cause, for example, disruption of color balance between the skin color portion of a face and the portion other than the skin color portion, such as the eyes or the like. Further, the technique described in U.S. Patent Application Publication No. 20060171605 performs calculation for exposure correction and white balance correction by changing the weight with respect to the main subject region and the region other than the main subject region and performs corrections on the recorded image. Here, only a single correction value is used for correction and a case where existence of a subject illuminated by a plurality of light sources in an image is not taken into account. Still further, the technique described in Japanese Unexamined Patent Publication No. 2003-189325 obtains information for white balance correction from the region other than the detected face region. It prevent unpleasant white balance causing the face portion to become pale or the like, but still a case where existence of a subject illuminated by a plurality of light sources in an image is not taken into account.
In the mean time, for a low sensitivity digital camera, performance of white balance correction with respect only to the main light source illuminating the main subject may result in an image with a uniform white balance over the entire image plane. For example, white balance correction with respect to the sunlight if outdoor environments or with respect to flash light if indoor environments at night and the flash is used may result in a favorable image. The reason is that the low sensitivity digital camera is unable to capture a weak light amount from a light source other than the main light source. On the other hand, in photography with a high sensitivity digital camera, the digital camera may capture both light amounts from the main light source illuminating a main subject and from a subsidiary light source illuminating the background thereof. In a case where a main subject is imaged with a flash as the main light source under an incandescent lamp light source as a subsidiary light source, the background illuminated by the subsidiary light source is also reproducible. In this case, the flash light as the main light source has a color temperature of about 7000K, while the incandescent lamp as the subsidiary light source has a color temperature of about 3000K. Thus, if the white balance over the entire image is adjusted to the flash light as the main light source, the background illuminated by the subsidiary light source becomes reddish. Further, when the subsidiary light source is an incandescent color fluorescent lamp or the like, a similar problem still arises. The present invention has been developed as a means for solving the problems described above.